1. Field of the Invention
The present invention relates to non-reciprocal circuit devices, and, more particularly, to non-reciprocal circuit devices, such as isolators or circulators, for use in the microwave band.
2. Description of the Related Art
In general, non-reciprocal circuit devices, such as isolators or circulators, have a characteristic of transmitting a signal only in a given direction but not in the opposite direction. By utilizing this characteristic, for example, isolators are used in transmitting circuits of mobile communication devices, such as automobile phones and cellular phones.
In order to protect a ferrite in which a center electrode is provided and an assembly of permanent magnets that apply a DC magnetic field to the ferrite, a non-reciprocal circuit device of the type described above has a structure in which the periphery of the assembly is surrounded by an annular yoke (see, for example, International Publication No. 2006/011383) or a box-shaped yoke (see, for example, Japanese Unexamined Patent Application Publication No. 2002-198707).
However, in a known non-reciprocal circuit device, an annular yoke or a box-shaped yoke obtained by forming soft iron or the like into an annular shape is used as a magnetic-shielding component. Thus, the soft iron formation processing or assembling takes time, resulting in a high cost. Moreover, since the yoke is present in the periphery of the ferrite or the permanent magnets, the size of the outer shape of the non-reciprocal circuit device itself increases. Or, when the increase in the size of the outer shape is avoided, the size of the ferrite or the permanent magnets decreases, which causes a problem of deteriorating electrical characteristics. This is caused by the fact that the reduced-size ferrite causes the size of the center electrode to be reduced, which reduces an inductance value or a Q value.